Differences exist in speed of breakdown and the number of electrons needed to initiate a self-sustained discharge, but the underlying breakdown mechanism is the same for low pressure discharges (e.g., fluorescent lamps) or high pressure discharges (arc discharge lamps). Discharge is initiated between two conductors that are given opposite electric potential. The space between the conductors usually comprises a gas, and efforts are made to maintain the quality/purity of the gas by enclosing it in a hermetic vessel. The essential end result of the discharge is the creation of a plasma between the two conductors. Plasma is defined as a conductive medium, containing equal proportions of electron and ions, which allows for conduction of electric current through an otherwise insulator material, i.e., the gas in its initial state.
Initially, the gas contained in the arc tube is non-conductive. If an electric potential is applied on the conductors, this creates a favorable situation to strip the outer orbital electrons from the atoms of the gas and thus create free electrons, which are then accelerated though the gas by the electric field generated between the conductors, and initiates more electrons by collision with gas atoms, which in turn are ionized. If the electric field is high enough, each electron thus created will create additional electrons by inelastic collisions with gas atoms and ions, and initiates an electron avalanche. Such an avalanche creates the discharge. However, to create such electrons by simple dielectric breakdown of the gas atoms by the electric field requires several kilovolts of electric potential. Higher and higher electric potentials require more expensive external electrical circuitry, and may not be commercially feasible. Unwanted breakdown can also occur in the outer jacket and in the cap-base region.
Discharges for commercial applications employ an additional source of free electrons, which removes the need for generating such high voltages to initiate the discharge. Such external sources can be a heated filament, use of the ever present cosmic rays, or providing a source of electrons by radioactive decay. Heated filaments are not practical in high intensity discharge (HID) lamps, and the cosmic ray background radiation is insufficient to dramatically reduce the need for very high electric fields needed to initiate the ignition, unless other methods are used to lower the breakdown voltage.
For providing a source of electrons by radioactive decay, typically what has been used in the past in the HID arc tube is a radioactive gas, such as Kr85 with most of the decay products being beta particles (i.e., electrons). Kr85 has a half-life of 10.8 years, with 99.6% of the decay products being beta particles (i.e., electrons) having a maximum kinetic energy of 687 kev. These electrons have very high energy, and in many respects are an ideal source for free electrons and used widely as such for these applications. But to provide enough of these high energy electrons by radioactive decay, significant quantity of this gas has been used in HID lamps.
The presence of Kr85 in such lamps diminishes the need for providing very high electric potential on the conductors, which makes the external electrical circuitry (a ballast) and systems design simpler and more cost effective. Typical applications use such a radioactive gas with a ballast that provides a high electric pulse for a very short duration, typically in the millisecond (microsecond) range, that is very effective in creating the electron avalanche referred to earlier. However, recent UN2911 government regulations limit the amount of radioactive Kr85 used in lamps. These regulations proscribe the HID lamp manufacturers from using the large quantity of Kr85 gas that has been previously used, as described in preceding paragraph.
A number of ignition aids have been designed for improving the ignition of high intensity discharge lamps. U.S. Patent application Pub. No. 2002/0185973 discloses a lamp in which wire is wrapped around both legs of the arc tube and its central body as both an ignition aid and for containment, but are not connected to the electrodes. Another reference, U.S. Pat. No. 5,541,480, discloses an ignition aid in which a conductor that is coated on an exterior surface of an arc tube of constant diameter between the electrodes is connected to a conductive frame wire that contacts an electrode. U.S. Pat. No. 6,222,320 discloses an ignition aid for a lamp including an arc tube having a central body portion and smaller diameter legs extending from the body portion, wherein a conductor that is in contact with a conductive frame wire that contacts one of the electrodes, contacts only the central body portion of the arc tube.